Load-bearing adaptation of women’s spines

Those of you who have been pregnant, or have been a partner to someone who has been pregnant, are familiar with one among many common consequences: lower back pain. It's not surprising—pregnant women are carrying this low-slung 7kg (15lb) weight, and the closest we males can come to the experience would be pressing a bowling ball to our bellybutton and hauling it around with us everywhere we go. This is the kind of load that can put someone seriously out of balance, and one way we compensate for a forward-projecting load is to increase the curvature of our spines (especially the lumbar spine, or lower back), and throw our shoulders back to move our center of mass (COM) back.

Here's the interesting part: women have changed the shape of individual vertebrae to better enable maintenance of this increased curvature, called lordosis, and fossil australopithecines show a similar variation.

14 Comments

I have to congratulate PZ on his primary objection, that this may very well be epigenetic plasticity. The case is also illustrates pretty clerly how an epigenetic hypothesis about an adaptation can be a TRUE alternative to a hypothesis of gene selection. Ultradarwinists tend ignore the fact that truly dichotomical alternatives can exist when examining real adaptations.
I would like to point out that such epigenetic origin of and adaptation is AS MUCH a part of evolution as gene selection, but unfortunately people think they only have a proper evolutionary explanation in the second case…

For the most part, epigenetic has come to mean the transmission of a trait controlled by a mechanism not encoded in the DNA sequence itself. An example is cytosine methylation, a reversible covalent modification of DNA that is usually associated with gene silencing. Certain covalent modifications of histones, such as methylation at lysine 9 of histone H3 (a mark associated with heterochromatic silencing) and lysine 27 of histone H3 (a mark associated with homeotic gene silencing) are often considered as epigenetic modifications, too.

Joel,
Interesting. That sounds sort of like an on/off switch that can be passed down with its setting intact. I recall hearing of species that switch between two or a few different forms every generation or every few generations - would something like this be involved in that?

Alexander Vargas,
Even if an “ultradarwinist” might tend to ignore, or might not know about, such things, is it likely that a large fraction of biologists would be unaware of (or disregard) those possibilities?

I would be curious to know how these differences appear within families that have a history of being overweight (because of some kind of genetic predisposition) from early childhood.

I’m not suggesting Lamarckism here. I’m wondering if families that have genetic predispositions for overweight also show such a trait in their vertebrae.

If you look at people carrying a lot of weight (especially belly fat), they often have sway-back posture (and many have back problems). And even though sumo wrestlers put their fat on differently and also have great strength, many appear to be sway-backed in their posture as well. Of course placement of the center-of-mass is behind the sway-back posture, that’s just physics.

But if this is a prolonged situation from early childhood, and it also runs in a family, is there a way to tell the difference between bones developing that way in response to the load and an evolutionary trait?

Apparently women have these differences in vertebrae from early childhood even though their backs have not carried awkward front loads for most of their development. On the other hand, if a person is overweight from early childhood, these changes in the vertebrae might develop as the child grows, and it could be similar in both male and female. The question is: what about families that have long histories of being overweight?

“I recall hearing of species that switch between two or a few different forms every generation or every few generations - would something like this be involved in that?”

It’s not the first hypothesis I’d entertain, but it’s not beyond the realm of possibility. The HOX genes that control axial segment identity (including the morphogenesis of vertebral segments) are subject to epigenetic control.

I would like to point out that such epigenetic origin of and adaptation is AS MUCH a part of evolution as gene selection, but unfortunately people think they only have a proper evolutionary explanation in the second case…

uh, perhaps you don’t fully understand how epigenetics works, but all the evidence points to epigenetic operators being heritable. after all, they are really just regulatory mechanisms.

suggest an excellent place to start looking at epigenetic research would be the recent Nova special on the subject:

I’m not sure what “epigenetic operators” are, but epigenetic decisions, while heritable mitotically and sometimes meiotically, are notoriously metastable, at frequencies that exceed–by orders of magnitude–those of genetic events.

The “eversporting” phenotypes in flies and the paramutable allele of maize are heritable, but that doesn’t make them “genetic.”

None of this negates Mendelism, of course. They are the exceptions that prove the rule.

Christians don’t object to the fact women have human babies, that’s not evolutionism! It would be evolutionism if she gave birth to a monkey or perhaps an X-man. Try again, this is not evidence for evolutionism. When Jesus said every creature was to reproduce after its own kind he meant it and there is no evidence otherwise!

I have a challenge for those creationists who claim that evolution is “only a theory which hasn’t been proven.” Will you promise to put your money where your mouth is and no longer use modern antibiotics and vaccines? I ask because we manufacture such medicines using the science of evolutionary biology; without our understanding of the evolution of living organisms, we wouldn’t have the drugs necessary to fight off MRSA and the Avian Flu, for instance. So for creationists who “don’t believe in evolution” to be true to their beliefs and not be hypocritical, they have to swear off these medical technologies and stick to good old penicillin the next time they become deathly ill.

So here’s the question, I wonder if we’ll see them avoiding the health clinics (which hand out “godless evolution” medicine) during the next epidemic, or will they rely on nothing but their faith to help them get better?

My Answer: There’s already a group of people who tried that - the Christian Scientists. Ever notice there’s not too many of them about nowadays? They may be mostly dead, but at least they aren’t hypocrites.

Any biologist who does his homework on the development of bone morphology is familar with the evidence that activity can affect bone shape, bone fusion, new ossifications within tendons, etc. (CERTAINLY much more than simple bone density)

“uh, perhaps you don’t fully understand how epigenetics works, but all the evidence points to epigenetic operators being heritable. after all, they are really just regulatory mechanisms”

Ichthyic, it’s pretty simple. Either the trait has been specifically selected for, or it is a fairly inescapable epigenetic correlation of combining normal bone development and the mechanical forces related to pregnancy and bipedality.

There are examples you should be familiar with, Icthyic. Such as the different adaptive morphologies of pharyngeal jaws in cychlids, which was found to vary according to the food you grow them. That transition, from one pharyngeal jaw morphology to the other, can occur without any new mutation or selection.